Lifting and leaning chairs, because of their adjustable heights, can be adapted to people having different heights and also to worktables of different heights for use, and can increase the level of comfort for people using them. As a result, their range of use is getting wider and wider and the number of people using them is becoming larger and larger.
The prior art such as the Chinese patent No. 200910249643.9 discloses a chair chassis with adjustable leaning force. It comprises a middle plate and a large plate. The rear part of the large plate can swing downwards relative to the middle plate around a rotary shaft. A regulating knob with a cavity is arranged under the front part of the middle plate. A mounting plate fixed with the regulating knob is arranged within the cavity of the regulating knob, and a worm wheel that is rotatable relative to the regulating knob around its own axis is arranged at the corresponding position of the regulating knob under the mounting plate. A worm engaging with the worm wheel is arranged within the regulating knob at the corresponding position beside the worm wheel. One end of the handle projects into the regulating knob from the outside of the regulating knob to be in transmissive connection with the worm. A regulating bolt, which in turn passes through the large plate, the middle plate and the mounting plate within the regulating knob from top to bottom, matches with the worm wheel by threads. The regulating bolt is coaxial with the worm wheel. A spiral spring is sleeved on the part of regulating bolt that is below the middle plate and above the mounting plate.
When a chair is leant, the spiral spring acts to reset the leant chair and at the same time regulates the extent of compression of the spiral spring by the worm wheel and worm structure in order to achieve the regulation of the chair leaning reset force. However, as the chair is being leant, the regulating bolt swings continuously within the mounting hole and at the same time the screw head of the regulating bolt is in continuous friction with the chassis, causing continuous wear for the chassis and even causing the easy occurrence of cracks at the mounting hole of the chassis during a long period of time in use. Therefore, it can be seen that the mounting hole of the chassis is not strong enough.
At the same time, many chairs also need to be exported to various countries all over the world at present, becoming more internationalized. Therefore, at present an international standard related to office chairs ANSI BIFMA arises, which has specific index and requirement for various performance of chairs, such as back pad static tension test, chassis lock test, chair cushion static shock test, chair cushion fatigue shock test as well as back pad fatigue test, etc. Among others, the test about the back pad fatigue is intended for evaluating the ability of the chair against the fatigue pressure and the wear caused by the force applied backward on the back pad. By applying a force in the vertical direction to the chair cushion and then applying a backward tilting force to the back during the test, the back is caused to tilt backward and reset continuously and cycle like this. During this course, the regulating bolt swings continuously within the mounting hole, causing the occurrence of cracks with the continuous wear at the mounting hole of the chassis. However in the BIFMA standard, a chair is qualified only if it repeats such test for 300 thousand times and no cracks appear, which sets very high requirement for the strength at the mounting hole of the chassis. At present, all chassises on the market process mounting holes directly, such as in the Chinese patent No. 200910249643.9 above. However, in order to undergo tests for up to 300 thousand times in the BIFMA standard, the general thickness of such chassis will be above 3 mm or more, which increases material for the products and further increases production cost. However, if the chassis is reduced below 3 mm in thickness in order to reduce cost, it in turn cannot pass the back pad fatigue test in the BIFMA standard. Therefore, there is a need for further improvement. At present, a difficult problem needed to be overcome is to study a chassis which can both save cost and pass the BIFMA standard.